1. Field of the Invention
The present invention relates to a process for preparing low-dusting, free-flowing dispersible pigments for use in paint and plastic compositions, and the pigments produced thereby. More specifically, the present invention relates to the aqueous milling, surface treatment, and spray-drying of inorganic pigments to achieve low-dusting and good dispersibility without the expensive step of micronization. Most specifically, the present invention relates to treating sandmilled titanium dioxide pigments with a surfactant before spray-drying the pigment, to eliminate the need for micronization or jet-milling while maintaining good processibility and dispersibility in polyolefin resins.
2. Description of the Prior Art
Titanium dioxide pigments and other inorganic pigments normally produced and used in the paint, plastics, or paper industry, are generally in the form of a finely divided powder. The powders are usually jet-milled, or micronized, as a final step in their production. Jet-milling contributes dispersibility and gloss, but is also an energy-intensive and expensive step. Jet-milled powders are inherently dusty and exhibit poor flow characteristics. Although free-flowing powders with low dust can be obtained by spray-drying, these powders generally exhibit poor pigmentary properties. Thus, pigment end users have generally had to choose between free-flowing, low-dusting, spray-dried pigments with poor pigmentary properties and dusty, jet-milled pigments with poor flow characteristics.
The surface treatment of pigments to achieve improved performance characteristics in plastic compositions, is known in the art. For example, U.S. Pat. No. 4,986,853 discloses the coating of lamina-shaped pearlescent pigments with monocarboxylic acids or cyclohexanone condensate resins and a plasticizer, to improve flowability and reduce dust generation. Additionally, U.S. Pat. No. 4,762,523 discloses coating a moist pigment with a polyester surfactant, then adding mineral oil or wax to the pigment and applying intensive shear stress to achieve a free-flowing, permanently non-dusting pigment. In U.S. Pat. No. 4,563,221, titanium dioxide pigments are coated with a combination of isostearic acid, dodecyl benzene sulphonic acid, and a cationic emulsifying agent and then spray-dried, without micronization, to improve flowability and reduce dust production. Also, in U.S. Pat. No.3,660,129, titanium dioxide pigments are coated with hydrous oxides, then sand-milled and spray-dried to improve flowability.
Further examples of the surface treatment or coating of pigments to improve processibility include the coating process disclosed in U.S. Pat. No. 4,514,231. This patent teaches the coating of an oxidic or silicate filler with an organosilicon emulsifying compound and then spray-drying to improve the reinforcing properties of the filler. Similarly, U.S. Pat. No. 4,909,853 teaches the aqueous coating of organic pigments and/or carbon black with a sulphosuccinic acid ester surfactant followed by spray drying to improve pigment dispersibility in thermoplastics. Also, U.S. Pat. No. 4,464,203 discloses the treatment of inorganic or organic pigments with an amine and ethylene oxide block copolymer surfactant to improve pigment dispersibility and other properties in a variety of applications. U.S. Pat. No. 4,156,616 discloses the treatment of inorganic and/or organic pigments with an alkylene oxide addition product of a long-chain amine and an anionic surfactant with an aromatic moiety, for improved dispersibility.
In U.S. Pat. No. 4,056,402, aqueous slurries of inorganic or organic pigments are milled in the presence of a nonionic, polyether alcohol dispersing agent and nonionic cellulose ether to reduce dusting and improve dispersibility. Also, in U.S. Pat. No. 3,947,287, aqueous, colored, inorganic and organic pigments are coated with the reaction product of a polyhydroxyl compound and a water retaining agent for improved stability and performance. U.S. Pat. No. 3,925,095 teaches the treatment of inorganic pigments or fillers with hydroxyalkylated alkylene diamine dispersing-agent for improved flowability and dispersibility in various applications.
Other examples of pigment surface treatment and coating processes include the treatment process described in U.S. Pat. No. 4,375,520. This patent teaches the production of dustless particles, including pigments, by treatment of the particles with a composition containing a solid low-molecular weight polymer and a liquid polymeric substance such as expoxidized soybean oil at temperatures above the melt point of the polymer. Also, U.S. Pat. No. 4,127,421 discloses the aqueous treatment of a lead chromate-containing pigment with a friable hydrocarbon resin and a cationic surfactant to produce free-flowing, non-dusting granules.
In U.S. Pat. No. 4,599,114, an aqueous slurry of titanium dioxide is treated with a surfactant formed from the reaction of a diamine, a carboxylic acid, and a fatty acid to improve pigment dispersibility and processibility in various applications. U.S. Pat. No. 5,228,912 teaches the surface treatment of platelet-shaped pigments with a polyacrylate, polymethacrylate, or salt thereof, for improved dispersibility in paints and printing inks. Also, U.S. Pat. No. 5,199,986 discloses a process wherein previously spray-dried inorganic pigment granules are coated with water and solutions of salts of boron, aluminum, silicon, titanium, zinc, brass, or tin, to improve processibility and reduce dust production.
U.S. Pat. No. 5,266,622 teaches the production of aqueous dispersions of inorganic or organic pigments or fillers wherein the pigments or fillers are coated with a water soluble polymer, a nonionic alkylene oxide adduct, and other dispersants, to improve flow properties and increase the stability of the pigment dispersion. Similarly, U.S. Pat. No. 4,186,028 teaches the coating of various pigments and fillers with a phosphonocarboxylic acid and/or a phosphonocarboxylic acid salt to produce fluid, dispersible, aqueous filler or pigment dispersions.
None of the aforementioned patents suggest or teach the process comprising milling an aqueous pigment slurry, treating the slurry with a treating agent, and spray-drying the slurry, which is the subject of this application.